How does Voyager still speak after all these years?

How does Voyager still speak after all these years?

Tech news channels have recently been abuzz with stories about strange signals coming from Traveler 1. While the usual suspects jumped to the usual conclusions – aliens!! – in the absence of a firm explanation for the anomaly, some of us viewed this event as an opportunity to marvel at the fact that the two Voyager spacecraft, now over 40 years old, are still in constant contact with us back on Earth, despite having traveled about 20 billion kilometers in one of the most hostile environments imaginable.

Like many NASA programs, Voyager far exceeded its original design goals and continues to yield useful science data to this day. But how is this possible? What radio technology from the 1970s enabled the twin space probes to not only fulfill their primary mission of exploring the outer planets, but also let them participate in an extended mission in interstellar space, while remaining in two-way contact? It turns out there’s nothing magic about Voyager’s radio – just solid engineering seasoned with a healthy dose of redundancy and a bit of luck over the years.

The big dish

For a program that in many ways defined the post-Apollo era of planetary exploration, Voyager was conceived surprisingly early. The mission’s complex profile originated in the mid-1960s “Grand Planetary Tour” concept, which was planned to take advantage of an alignment of the outer planets that would occur in the late 1970s. launched at the right time, a probe would be able to reach Jupiter, Saturn, Uranus and Neptune using only gravitational aids after its initial boost, before being thrown on a trajectory that would eventually take it into space interstellar.

The idea of ​​visiting all the outer planets was too enticing to pass up, and with the success of the Pioneer missions to Jupiter serving as dress rehearsals, the Voyager program was conceived. Like all NASA programs, Voyager had certain primary mission objectives, a minimum set of planetary science experiments that project leaders were reasonably confident they could accomplish. The Voyager spacecraft was designed to achieve these fundamental mission goals, but planners also hoped the vehicles would survive beyond their final planetary encounters and provide valuable data as they traversed the void. And so the hardware, both in the spacecraft and on the ground, reflects that hope.

Voyager primary reflector being manufactured, around 1975. The body of the dish is made of aluminum honeycomb and is covered with epoxy laminate skins impregnated with graphite. The surface precision of the finished plate is 250 μm. Source: NASA/JPL

The most important physical feature of the Deep Space Network (DSN) ground stations, which we have already covered in depth, and of the Voyager spacecraft itself are their satellite dishes. Although the scale may differ – DSN sports telescopes up to 70 meters in diameter – the Voyager twins were each launched with the largest dish that could fit in the fairing of the Titan IIIE launch vehicle.

Schematic of the Voyager High Gain Antenna (HGA). Note the Cassegrain optics, as well as the frequency-selective sub-reflector which is transparent to S-band (2.3 GHz) but reflects X-band (8.4 GHz). Click to enlarge. Source: NASA/JPL

The main reflector of the High Gain Antenna (HGA) of each Voyager spacecraft is a parabolic antenna 3.7 meters in diameter. The dish is made of honeycomb aluminum covered with an epoxy laminate skin impregnated with graphite. The surface of the reflector is finished with a high degree of smoothness, with a surface precision of 250 μm, which is necessary for use in both the S band (2.3 GHz), used for uplink and downlink , and in the X band (8.4 GHz), which is downlink only.

Like their terrestrial counterparts in the DSN, the Voyager antennas are a Cassegrain reflector design, which uses a frequency selective sub-reflector (FSS) at the focus of the main reflector. The sub-reflector focuses and corrects incoming X-band waves towards the center of the main dish, where the X-band feed horn is located. This arrangement provides approximately 48 dBi of gain and a beamwidth of 0.5 ° on X-band. The S-band layout is a bit different, with the feedhorn located inside the sub-reflector. The frequency-selective nature of the sub-reflector material allows S-band signals to pass through it and directly illuminate the main reflector. This gives about 36 dBi of gain in the S-band, with a beamwidth of 2.3°. There is also a low-gain S-band antenna with a more or less cardioid radiation pattern located on the earth-facing side of the sub-reflector assembly, but which was only used for the first 80 days of the mission.

two is one

Three of the ten bays on each Voyager bus are dedicated to radio frequency subsystem, or RFS, transmitters, receivers, amplifiers, and modulators. As with all high-risk space missions, redundancy is the name of the game – nearly every potential point of failure in the RFS has some sort of backup, an engineering design decision that has proven to save the mission in more than a case on two spacecraft over the past 40 years.

On the uplink side, each Voyager has two S-band double-conversion superhet receivers. In April 1978, barely a year before its scheduled encounter with Jupiter, the primary S-band receiver on Traveler 2 was shut down by crash protection algorithms on the spacecraft which did not pick up any commands from Earth for an extended period of time. The backup receiver was turned on, but found to have a bad capacitor in the phase-locked loop circuit intended to accommodate Doppler shift frequency changes due primarily to earth motion. Mission controllers ordered the spacecraft back to the primary receiver, but that again failed, leaving Traveler 2 with no means of being controlled from the ground.

Fortunately, the fault protection routines turned the backup receiver back on after a week without communication, but that left the controllers in a pickle. To continue the mission, they had to find a way to use the wonky backup receiver to command the spacecraft. They proposed a complex scheme in which DSN controllers estimate the uplink frequency based on the predicted Doppler shift. The problem is that, thanks to the bad capacitor, the signal must be within 100 Hz of the receiver’s lock-on frequency, and this frequency changes with receiver temperature, by about 400 Hz per degree. This means that controllers should run tests twice a week to determine the current lockout frequency, and also allow the spacecraft to thermally stabilize for three days after uplinking any commands that could alter the spacecraft’s temperature.

Double downlinks

Ken Shirriff

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An Apollo-era TWTA, similar to the S-band and X-band power amps used on Voyager. Source: Ken Shirriff

On the transmit side, X-band and S-band transmitters use separate exciters and amplifiers, and again, multiples of each for redundancy. Although the downlink is primarily through the X-band transmitter, one of the two S-band exciters can be fed into one of two different power amplifiers. A solid state amplifier (SSA) provides selectable 6W or 15W output power to the feedhorn, while a separate traveling wave tube amplifier (TWTA) provides 6.5W or 19W. Dual X-band exciters, which use the S-band exciters as a frequency reference, use one of two dedicated TWTAs, each of which can send 12W or 18W to the high-gain antenna.

The redundancy built into the downlink side of the radio system would play a role in safeguarding the primary mission of both spacecraft. In October 1987, Traveler 1 suffered a failure in one of the X-band TWTAs. Just over a year later, Traveler 2 encountered the same problem. Both spacecraft were able to switch to the other TWTA, allowing Traveler 1 to return the famous “family portrait” of the solar system, including the Pale Blue Dot image of Earth, and to Traveler 2 to return data from its flyby of Neptune in 1989.

Slower and slower

The Voyager systems’ radio systems were primarily designed to support planetary flybys, and so were optimized to broadcast as much science data as possible to the DSN. Close approaches to each of the outer planets resulted in each spacecraft dramatically accelerating during flybys, just at the time of peak data output from the ten science instruments on board. To avoid bottlenecks, each Voyager included a Digital Tape Recorder (DTR), which was essentially a sophisticated 8-track tape recorder, to buffer science data for later downlink.

Additionally, the increasing distance with each Voyager has greatly reduced the bandwidth available for downlink science data. When the spacecraft made its first flybys of Jupiter, data was broadcast at a relatively energetic rate of 115,200 bits per second. Now, as the spacecraft each approach a full light day, data is only flowing at 160 bps. Uplink commands are even slower, at just 16 bps, and are broadcast into space from the DSN’s 70-meter parabolic antennas using 18 kW of power. The uplink path loss over the current distance of 23 billion kilometers up to Traveler 1 exceeds 200 dB; on the downlink side, DSN telescopes have to dig a signal that has faded to attowatt (10-18 W).

That the radio systems of Traveler 1 and Traveler 2 worked at all while still in the main part of their planetary mission is a technical achievement worth celebrating. That the two spacecraft still communicate, despite the challenges of four decades in space and multiple system failures, is almost a miracle.

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